In the technique for determining base sequences in nucleic acids such as a DNA and a RNA according to the chemical degradation process, the dideoxy process or the like, slab electrophoresis using an electrophoresis medium membrane containing an aqueous polyacrylamide gel (hereinafter referred to as a polyacrylamide gel membrane or simply as a gel membrane) is indispensable. In recent years, electrophoresis analysis has come into wide use. Also, with the advances made in the dideoxy process, there has arisen a need for a polyacrylamide gel membrane capable of accurately separating up to a high molecular part of fragments of a nucleic acid.
On the other hand, in the case where fragments of a nucleic acid are electrophoretically separated based on a difference in its molecular weight for the base sequence determination of the nucleic acid and an ordinary polyacrylamide gel membrane is used for this purpose, the band intervals of the separated fragments become wider for a low molecular part and narrower for a high molecular part. As a result, the separation of the high molecular part of the nucleic acid fragments is deteriorated.
Also, an electrophoretic separation and fractionation technique has heretofore been widely utilized for the separation and fractionation analysis of constituents of a living organism. Particularly, the separation and fractionation analysis of proteins is frequently utilized in biochemical inspection for diagnosis of diseases. In the case where a protein is separated and fractionated by a single electrophoretic operation based on a difference in its molecular weight and an ordinary polyacrylamide gel membrane having constant concentration is used for this purpose, the band intervals of the separated and fractionated protein become wider for a low molecular part and narrower for a high molecular part. As a result, the separation of the high molecular part of the protein is deteriorated.